The invention relates to a membrane assembly for electrochemical processes, and to processes that may be practiced utilizing the membrane assembly of the invention, or a membrane assembly having like properties. An important feature of the membrane assembly of the present invention is that it allows the passage of anions therethrough, while effectively retarding the passage of water therethrough. The membrane assembly according to the present invention is capable of continuous long-term operation. The term "extended operation", as used in the present specification and claims is intended to mean successful operation over at least hundreds of hours, if not months and years.
In the past there have been numerous proposals for anion permeable membranes for use in electrochemical cells, such as "amberlite" resins, formed by bonding together resin particles, beads or granules. Such membranes, and like ion exchange membranes, are not capable of extended operation, fouling quickly, and require the application of such a large amount of energy as to be totally impractical, can easily be clogged, can react chemically with ions transmitted thereby, are highly susceptible to radiation damage, pass hydration water, are subject to swelling and/or are permselective.
According to the membrane assembly of the present invention, however, it is possible to provide a membrane which avoids the disadvantages of prior art anion-selective membranes and additionally is superior in many respects to conventional cation-permeable membranes. The membrane assembly, according to the present invention, is anion-permeable; it will not readily pass water; allows cation and anion cross-flow (is not permselective); is inexpensive, simple to make, and uses inexpensive and easy-to-make materials without any special processing; is generally not degraded by radioactivity, providing generally excellent radioactive ion separation; does not swell as a result of chemical reactions therein; is not susceptible to clogging or ready degradation; does not have a membrane potential (is free of anions and cations), therefore does not affect an increase in the water head on one side of the membrane (with associated overflow problems), except for very weak electrolytes; has increased electrical efficiency (high Faraday current efficiency with low voltage drop); and can be constructed of a variety of materials and configurations, therefore the exact material and design parameters can be chosen to exactly fit a particular situation. Additionally, utilized in special forms, all back-diffusion is prevented, and when utilized in particular cell configurations, has a mass-transfer efficiency of well over 100% (e.g. 170%), caused by the combination of electrolytical current-efficiency and electro-pherotic effects.
The membrane assembly according to the present invention comprises a plurality of layers of substantially void-free capillary material, each layer being thin enough so that it retards the passage of water therethrough, and thick enough so that it allows passage of ions therethrough, the capillary material having a positive angle of wetting; and means for forming the capillary material into directed capillary channels allowing passage of anions and cations therethrough, and means for preventing cross-flow of water and ions between the capillary channels. The forming and preventing means preferably comprises a plurality of separation layers of inert and impermeable material, having a high dielectric constant and smooth surfaces, and means for maintaining the capillary and separation layers in position so that the separation layers are interposed with the layers of capillary material, all of the layers being substantially parallel, so that anion and cation transport across the assembly through capillary material layers takes place, but ion and water transport across the separation layers does not take place. The capillary material may be chosen from a wide variety of conventional capillary materials, including paper, asbestos, wood, synthetic felts, and polyester and polypropylene woven or non-woven webs, and normally will be chosen so that it has a thickness of about 0.001 to 0.03 inches. In order to decrease the voltage drop, while still providing proper functioning, the capillary material normally has a dimension in the direction of ion transport of about 0.1 to 1.5 inches. The separation layers may be comprised of a wide variety of conventional inert and impermeable material, having high dielectric constant, such as non-conducting plastic films (preferably polyethylene film), rigid plastic plates, ceramic and glass.
In order to prevent essentially all back-diffusion, and for other purposes, a membrane assembly according to the present invention may be provided in combination with a second said assembly and a film membrane assembly, such as described in my patent application Ser. No. 814,715, filed July 11, 1977, now U.S. Pat. No. 4,124,450, the disclosure of which is hereby incorporated by reference in the present application. The film membrane assembly comprises a film of substantially water-impermeable, ion-impermeable, insulating material having a thickness of about 0.001 to 1 mm. That combination includes means (e.g. glue) for maintaining the film membrane assembly sandwiched between the capillary assemblies, with the capillary assembles in intimate contact with the film and with the film being disposed substantially perpendicular to the direction of ion transport across the capillary assemblies. The capillary membrane assemblies disposed on either side of the film membrane comprise means for providing a continuation of the boundary layer thereof and for eliminating membrane potential and, thus, fouling of the membrane.
According to the present invention, electrochemical cells are provided comprising an anode, disposed in an anode chamber; a cathode, disposed in a cathode chamber; and a membrane assembly, disposed between the anode and the cathode, the membrane assembly comprising an anion permeable, semi-permeable or water impermeable, non-permselective membrane, capable of providing passage of anions thereacross over extended operation without destructive swelling, clogging, chemical reaction, or consumption thereof. The various chambers and membranes can have a wide variety of configurations. For instance, the membranes may be planar, curvilinear in the dimension perpendicular to ion transport (i.e. cylindrical or tubular), curvilinear in the direction of ion transport (i.e. waviform), and may may be provided as a large membrane with the anode and cathode chambers defined by cutouts in the membrane (the areas of the membrane defining the anode and cathode chambers being treated so that they are effective to prevent diffusion of materials from the anode and cathode chambers thereinto).
An exemplary cell according to the present invention may comprise a dialysis cell, including at least two membranes disposed between the anode and the cathode, and defining at least one central chamber therebetween, at least the membrane adjacent the anode chamber comprising a said anion permeable membrane. In order to lower the resistivity of the cell, and for acting as an electrode, and for acting as a barrier to water and metal transport when no current is applied to the anode, a plurality of graphite particles may be disposed in at least one central chamber. Three central chambers preferably are provided defined by at least three of said anion permeable membranes, each of the central chambers including a plurality of graphite particles disposed therein. In utilizing such a dialysis chamber, it is possible to obtain mass transfer efficiencies of over 100% (e.g. 170%)!
According to another method of the present invention, it is possible to form uranium and vanadium rich liquid from conventional phosphoric acid contaminated with the same. Conventional phosphate rock and overburden contain from 50 to 200 ppm of uranium, and the uranium is also found in the phosphoric acid produced from the ore in the production of fertilizers, and the like, and is found in the gypsum pond water from the processing plant. According to the present invention, the phosphoric acid at any concentration below 100% is selected from some convenient point in the process (i.e. after filtration or just before evaporation) and is subjected to electrodialysis. In addition to producing uranium and/or vanadium rich liquids according to the present invention, aluminum, iron, magnesium, calcium, and the like are removed--resulting in a better grade of metals and fluorine depleted--phosphoric acid--and gypsum pond water, which heretofore has been a significant disposal problem is even harmlessly disposed of. In fact, even food-grade phosphoric acid can be produced. According to the method of uranium and/or vanadium removal of the present invention, an electrochemical cell is utilized comprising an anode chamber defined by an anion permeable membrane, a cathode chamber defined by a cathode permeable membrane, and at least one central chamber between the anode and cathode chambers. Water is originally provided in the anode chamber and gypsum pond water in the cathode chamber. Contaminated phosphoric acid is fed into the central chamber, a current is supplied to the anode sufficient to effect electrodialysis of the contaminated phosphoric acid, the uranium and/or vanadium rich supernatant liquid is withdrawn from the cathode chamber, and the metal-depleted phosphoric acid is withdrawn from the anode chamber. Where a plurality of central chambers are provided, each having a plurality of graphite particles disposed therein and defined by anion permeable membranes, food-grade phosphoric acid may be withdrawn from the anode chamber.
According to another method of the present invention, it is possible to effectively and efficiently, over extended operation, demineralize water by subjecting mineral-containing water to electrodialysis. The mineral-containing water may be hard water, sea water, salt water, or brackish water. Again, a dialysis cell as described above is utilized, and the method is practiced by providing water as the anolyte and catholyte, feeding the mineral-containing water into the bottom of the central chamber. Metal-containing liquids and metal precipitates are also produced in the cathode chamber, while halogens and acids are produced in the anode chamber.
According to a still further method of the present invention, it is possible to remove chrome from chrome-contaminated rinse water from chromium plating operations, etching liquid, or cooling tower effluent. It is possible to effect chrome removal to such an extent that the water produced has less than 0.05 pp. chromium, and the chromium is concentrated to a sufficient extent so that it is recoverable and/or reusable (e.g. chromic acid having a concentration of about 15% or more can be produced). The method is practiced utilizing an electrolysis cell which may comprise a plurality of series-connected cathode chambers being separated by an anion permeable membrane, and the method being practiced by feeding chromium-contaminated water to the bottom of the first cathode chamber, supplying current sufficient to effect electrolysis to the anode, withdrawing water having less than 0.01 ppm (i.e. 0.05 ppm) chromium from the top of the last cathode chamber, and withdrawing concentrated chromic acid from the anode chamber.
Further processes practiced according to the present invention include the treatment of simple metal in a recoverable form, by electrolysis, production of chlorine and/or metal from a chlorine-containing salt and water, treating ECM sludge to produce nitric acid and sodium hydroxide, by electrodialysis, and treating phosphate slime (clay and phosphate rock mixed together) to produce phosphoric acid.
It is the primary object of the present invention to provide an improved membrane assembly, and processes that are capable of efficiently and effectively, over extended operation, purifying phosphoric acid while producing uranium and/or vanadium rich liquids, demineralizing water, removing chromium from chromium-contaminated water, turning ECM sludge and other milling wastes into reusable components, producing chlorine, and producing phosphoric acid from phosphate slime. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.